Books You Read That You've Never Identified

I have 2 that come to mind. And they both kind of scared me as a kid.

Both were children's picture books probably from the 2000's.

First one had only human characters and a very simplistic art style, drawn like a children's drawing, not really any shading or anything just black outlines and flat colours from what I remember. The plot had something to do with fruit juiceboxes (which I think they used the word "fruit punch" to refer to it because it was the first time I ever remember hearing that term), and there being bullies at school or someone steals the juice box? Not sure. But there's a girl who either bullies someone or otherwise does something wrong and she gets punished by being strapped to a bed (not joking), and she grows a devil tail and horns, but someone interrupts this and saves her.

Second one, I distinctly remember it being about a monster who looked vaguely similar to the monsters from Where The Wild Things Are (particularly in the live action movie) or The Gruffalo, but it was neither of these franchises. It was drawn in a kind of semirealistic art style I guess, and the only thing I really remember is that both the front and back cover of the book were the monster, so like the front cover of the book was the monster and the back cover of it was his back. I don't remember much about the plot or storyline except it had something to do with the monster being a typical "monster under the bed", hiding and watching you. It definitely gave me nightmares as a kid.